In the contemporary medical diagnostic field, a laser imager which scans a photographic material with laser light for recording has been used as a system for recording an image output from a diagnostic apparatus such as X-ray CT and MRI (nuclear magnetic resonance apparatus). This laser imager system, desirably, would be capable of recording and developing an image in a shorter period of time to provide a rapid diagnosis.
As recording lasers used for such a laser imager known recording laser for this purpose have a wavelength of 600 to 700 nm, such as He-Ne laser, and another recording layer having a wavelength of 700 to 850 nm, such as semiconductor laser.
Various properties are required by photographic materials for use in the laser recording system. In particular, since exposure is effected in a period of time as short as 10.sup.-3 to 10.sup.-7 second in this system, a relatively soft contrast recording material has been required in order to provide a high sensitivity and a sufficient black density and reduce uneven exposure upon a rapid processing under these conditions.
Various methods for the purpose of providing a sufficient black density in a short time development process have been known. Examples of these known methods include a method which comprises adjusting a gelatin crosslinking agent (film hardener) so as to increase the swelling of an emulsion layer in the development process; a high temperature rapid processing method which substantially prevents a developer from hardening gelatin (as disclosed in JP-A-63-136043 (The term "JP-A" as used herein means an "unexamined published Japanese patent application")); and a method which comprises the addition of a polymer such as acrylamide and dextran compound (as disclosed in U.S. Pat. Nos. 3,271,158, 3,514,289, 3,063,838, 3,272,631). However, these methods are disadvantageous in that the photographic material is dried slowly or the resulting film strength is poor. Alternatively, method which comprises the enhancement of the development activity of the processing solution is also known. To this end, the content of a developing agent or auxiliary developing agent in the developer may be increased. Alternatively, the pH value of the developer may be increased. Further, the processing temperature may be raised. However, these approaches are all disdavantageous in that the preservability of the processing solution is impaired or fog can easily take place.
Moreover, a method which comprises the concentration of development center of halide grains to enhance the development activity has been known. For example, a method which comprises doping an iridium compound during the formation of grains (as disclosed in Japanese Patent Application No. 3-266934) may be used.
However, the use of an iridium compound produces adverse side effects such as sensitivity drop and contrast hardening. In particular, silver halide emulsions which can be rapidly developed tend to provide a high contrast. Therefore, if the recording material used exhibits too high a gradation, scan line due to laser exposure or unevenness can easily take place, making it difficult to put this approach into practical use.
Thus, it has recently been desired to provide a photographic material which exhibits a sensitivity enough for a laser imager, an excellent adaptability to be rapidly processed and an appropriate gradation.
On the other hand, a method which comprises finely dividing silver halide emulsion grains for the purpose of obtaining a high density with a small coated amount of silver and hence enabling a rapid processing has been well known by those skilled in the art. However, this method is disadvantageous in that, as the size of silver halide emulsion grains is reduced, the surface gloss of developed image portion grows, causing a room lamp to be reflected and hence making the recorded image difficultly observable. Thus, photographic materials satisfying all these requirements have never been realized heretofore.